South African Olympic and Paralympic sprinter Oscar Pistorius sobbed in the witness stand on Tuesday as he described how, gripped by fear, he shot dead his girlfriend through a locked toilet door thinking she was an intruder.

Taking the witness stand, the double amputee — accused of murdering 29-year-old graduate and model Reeva Steenkamp — recounted hearing a window sliding open in his bathroom in the middle of the night on Valentine’s Day 2013.

Conditioned by years of living in crime-ridden South Africa, Pistorius said the noises convinced him someone was breaking into his luxury Pretoria home and that he needed to protect himself and his lover.

“That’s the moment that everything changed,” he said, his voice tense with emotion. “I thought that there was a burglar that was gaining entry to my home.”

“The first thing that ran through my mind was I needed to arm myself, that I needed to protect Reeva and that I needed to get my gun,” said the 27-year-old, who faces life in prison if convicted of murder.

Pistorius said he felt in the dark with his fingers, grabbed a 9mm pistol lying under the bed, then moved on his stumps down the passageway leading from the bedroom towards the bathroom and the would-be intruder or intruders.

“I shouted for Reeva to get on the floor and phone the police. I screamed at the people to get out,” he said.

According to his account, as he peered round the door of the bathroom – his outstretched right hand holding the pistol, his left hand steadying himself against the wall – he noticed the bathroom window was open, confirming his worst fears.

“I just stayed where I was and kept on screaming. Then I heard a noise from inside the toilet that I perceived to be somebody coming out of the toilet. Before I knew it, I had fired four shots at the door.”

With his ears ringing, he continued to scream for Steenkamp to call the police, he said, until it slowly dawned on him that his girlfriend might have been behind the door.

He then recounted how he bashed out a panel of the wooden door with a cricket bat to reveal Steenkamp slumped on the floor in a pool of blood.

“I sat over Reeva and I cried,” he said, before breaking down into uncontrollable sobs, causing Judge Thokozile Masipa to adjourn the hearing for the day.